In the transmission networks of optical communications, single-mode optical fibers are used. As optical communications using the single-mode optical fiber, wavelength division multiplexing transmission (WDM transmission) is widely accepted. Wavelength division multiplexing transmission is a method of transmitting a plurality of optical signals using a plurality of wavelengths, not one wavelength, for optical communications, which is an optical transmission method suited to large-capacity high-speed communications.
In wavelength division multiplexing transmission, in order to reduce wavelength dispersion, a dispersion shifted optical fiber is sometimes used. Among others, a non-zero dispersion-shifted fiber (NZDSF) is sometimes used, in which dispersion caused by wavelengths of light is reduced as well as the dispersion is not zero.
Patent Literature 1 below describes a dispersion shifted optical fiber. The dispersion shifted optical fiber described in Patent Literature 1 below includes a core having three segments.    [Patent Literature 1] JP4393708 B2